1. Field of the Invention
This invention relates to a control system, and more particularly, to a control system which is required to have a good robustness to disturbances, such as a control system for a motor.
2. Description of the Prior Art
In conventional control systems, a PI controller (Proportional-Integral controller) has been widely used in order to improve the robustness to disturbances, or to suppress influences of disturbances. Recently, the PI controller can provide a digital control using a microprocessor. For example, a digital PI controller using a microprocessor for controlling the speed of a DC motor is disclosed in "Digital Control Systems", chapter 14, section 14.4, pp 689-694, written by Benjamin C. Kou and published as Holt-Saunders International Editions.
The PI controller, whether analog or digital, can improve the robustness to disturbances more at a lower frequency. However, the recent requirement to improve the robustness to disturbances has become more and more severe in some applications. For example, a control system for controlling the speed of a capstan motor in a video tape recorder requires a highly improved robustness to torque disturbances, because the size and the inertia of the capstan motor have been greatly minimized in recent years.
We have developed new control systems for improving robustness to disturbances, which are disclosed in my previous U.S. patent application, Ser. No. 917,498 filed October 10, 1986 (now U.S. Pat. No. 4,821,168) and Ser. No. 22,872 filed March 6, 1987 (now U.S. Pat. No. 4,755,729). Each of these improved control systems has an excellent robustness to disturbances by using some RAM (random access memory) devices. However, there remains a problem in that each of these improved control systems requires a long period of time to reach a steady state of the improved robustness to disturbances from the starting timing of the control operation.